The teratogenic effect of a drug is highest during the first three months of pregnancy, except the first ten days after conception where if it is toxic for the fetus it causes an abortion .

A non teratogenic drug taken during the second or third trimester of pregnancy can induce disorders in the child, for example ACE (angiotensin converting enzyme) inhibitors can give oligohydramnios, growth retardation, renal insufficiency and even dead. Ace inhibitors should be avoided throughout pregnancy.

When prescribing a drug to a woman in child bearing age, the doctor must question about a possible pregnancy. If a prescription is necessary, he should choose in the therapeutic class he is intended to prescribe an old drug which has been already frequently used by pregnant women without adverse effects.